Top Army general: These 4 nations are the most dangerous to US security

Gen. Mark A. Milley, chief of staff of the Army, in the Army portrait studio at the Pentagon in Washington, D.C., on August 12. US Army/Monica King On June 20, Gen. Mark Milley, the US Army chief of staff, ranked four nations as the most dangerous to US national security: Russia, China, Iran, and North Korea.

Russia

It is the only country that is "literally an existential threat," Milley said at an Association of the US Army breakfast, adding that Russia could physically destroy the US. In addition, the country has postured itself as an aggressive state over that past several years.

Milley cited the country's response to Crimea, the attack on Georgia, and the increase in defense spending on modernization efforts as posturing. He also suggested that Russia was invading sovereign nations in a way that has not been seen since 1945.

China

Milley suggested that while Russia is aggressive, China is "assertive."

While its navy has begun exploring disputed waters, the country has made no attempts to venture into other sovereign nations. In addition, the China is not our enemy … yet.

Soldiers of China's People's Liberation Army navy in the Spratly Islands, known in China as the Nansha Islands, on February 10. Reuters

"I would caution anyone from saying China is an 'adversary,'" he said.

The issue is the long-term potential. Its economic growth and increasing military power suggest a steady rise and may indicate a major power shift, which could be dangerous in the future.

Though he did not dwell on the topic, Milley said, "There's no doubt Iran is a malign actor."

North Korea

Yes, everyone knows it has a small military and a petulant leader and is a resource-depleted country. But North Korea does have nuclear capabilities. All four characteristics make for an unstable nation with very little to lose. Plus, North Korea is backed by China in most instances.

North Korean leader Kim Jong Un at the Workers' Party Congress in Pyongyang on May 7 in this handout photo provided by KCNA. KCNA/via REUTERS

Though the threats of nuclear destruction have all proved to be empty words said by paper tigers, that doesn't mean they always will be.

"Just because it didn't happen before," Milley said, "is not a guarantee it won't happen tomorrow."